In telecommunications systems that are designed to handle signals with high peak-to-average power ratios (PAPR), Doherty power amplifier architectures have become popular due to their relatively high linearity and efficiency at back-off levels, when compared with other types of amplifier topologies. A conventional two-way Doherty power amplifier includes a class-AB biased main (or “main”) amplifier and a class-C biased peaking amplifier in a parallel arrangement. When an input signal has relatively low to moderate power, the main amplifier operates to amplify the input signal, and the peaking amplifier is minimally conducting (e.g., the peaking amplifier essentially is in an off state). During this phase of operation, an impedance transformer in the output combiner network determines the maximum VSWR (voltage standing wave ratio) to which the main amplifier will be exposed. Conversely, as the input signal power increases to a level at which the main amplifier reaches voltage saturation, the input signal is split (e.g., using a 3- or other decibel (dB) power splitter) between the main and peaking amplifier paths, and both amplifiers operate to amplify their respective portion of the input signal. Ultimately, the amplified signals are combined at a summing node to produce the final amplified output signal. It is important that phase coherency of the main and peaking RF signals is present when the signals reach the summing node, so that the main and peaking RF signals may combine in phase.
As the Doherty amplifier input signal level increases beyond the point at which the main amplifier is operating in compression, the peaking amplifier conduction also increases, thus supplying more current to the load. In response, the load line impedance of the main amplifier output decreases. In fact, an impedance modulation effect occurs in which the load line of the main amplifier changes dynamically in response to the input signal power (i.e., the peaking amplifier provides active load pulling to the main amplifier). An impedance inverter, which is coupled between the output of the main amplifier and the summing node, ensures that the main amplifier sees a high value load line impedance at backoff, allowing the main amplifier to efficiently supply power to the load over an extended output power range.
According to the operating principles of a Doherty amplifier, the impedance inverter in the main path should establish the correct load modulation characteristic. The impedance inverter has an electrical length of 90 degrees at the band center frequency, fo, and an associated group delay. Unfortunately, RF bandwidth limitations associated with differences in group delay in the main and peaking output paths may result in a loss, at frequencies other than fo, of phase coherency between the main and peaking currents received at the summing node. This loss of phase coherency may result in dispersion of the load impedances seen by the main and peaking amplifiers, along with a non-ideal load modulation over frequency. The primary outcome of operating with non-ideal load modulation is a loss of peak power capability over the full band of operation, or an effective reduction in the utilization of the Doherty power amplifier. This, in turn, may impact attainable efficiency performance for a fixed output power back-off level.